The present application is a U.S. national phase application of PCT International Application No. PCT/GB99/03427, having an international filing date of Oct. 22, 1999 and claiming priority to Great Britain Application No. 9823046.9 filed Oct. 22, 1998. The above PCT International Application was published in the English language and has International Publication No. WO 00/25321.
The present invention concerns electrochemical dissolution. By way of example, the present invention relates to the reprocessing of irradiated nuclear fuel and, by way of particular example, to the dissolution of nuclear fuel assemblies. Reference will be made hereinafter to the dissolution of nuclear fuel assemblies but it should be understood that the present invention has application to the dissolution of other structures including those which are radioactive as well as non-radioactive structures.
A nuclear fuel assembly typically includes a plurality of fuel rods or pins which are assembled together within a skeleton of grids. Each fuel rod or pin may typically be in the form of a carrier tube made of a suitable material such as the zirconium metal based alloy known as Zircaloy. Within the carrier tube is located a xe2x80x9cstackxe2x80x9d of nuclear fuel pellets.
In a known commercial method of reprocessing of irradiated (spent) nuclear fuel, the pins of an assembly are chopped up prior to dissolution of the uranium dioxide fuel pellets in nitric acid. The pins must be chopped up to expose the pellets to nitric acid because the bulk zirconium alloy is resistant to attack by nitric acid, as is an oxide skin or film which is present on the irradiated zirconium alloy. The chopping up of the pins is undesirable because it requires mechanical apparatus which becomes subject to serious wear and therefore requires relatively frequent repair. It will be appreciated that there are difficulties in repairing machinery which is used for the processing of radioactive material.
In a period between the 1950s and 1970 considerable experimental work was carried out on the electrochemical dissolution (xe2x80x9cECDxe2x80x9d) of complete (unchopped) fuel pins. A review of the development of ECD up to 1970 can be found in V P Caracciolo and J H Owen, progress in nuclear energy, Series III. Vol IV. Pergamon Press 1970, pp 81-118. The principle of ECD is that a fuel pin is placed in nitric acid and a potential difference is applied between the fuel cladding and the nitric acid surrounding it. If this potential is large enough then the inert nature of the cladding is overcome and it becomes reactive to the nitric acid.
Caracciolo and Owen describe a process in which the fuel pins are placed in wedge-shaped baskets whose sides taper towards each other towards the bottom of the basket. The basket is placed in nitric acid and is connected to a power supply. However, as the dissolution process is an oxidative one, contact between the metallic basket and the oxidising assembly is lost resulting in cessation of the process. Furthermore, the process has only been demonstrated on stainless steel clad fuel and would not work with Zircaloy clad fuel because the oxide film prevents electrical contact between basket and fuel assembly.
In addition to the problem posed by the pre-existing oxide film, there is also the problem that, due to the large number of fuel pins in an assembly, it is difficult to form adequate electrical contact to all the pins during the process. A pressurised water reactor (PWR) assembly can include up to 324 pins in an 18xc3x9718 matrix assembly. Attempts made to form direct electrical contact with the fuel assembly have all failed due to the problem of forming contact all pins in the assembly.
According to the present invention there is provided a process for the electrochemical dissolution of a metallic structure having a plurality of electrically conducting components, the process comprising utilising the structure as a sacrificial electrode in an electrochemical cell so as to dissolve at least a part of the structure, characterised in that, prior to use of the structure as a sacrificial electrode, molten metal is allowed to solidify about the structure so as electrically to connect together said components.
In the case where the metallic structure is a fuel assembly, one end of the assembly is lowered into a vessel containing molten metal. The molten metal is allowed to cool as a result of which a block of metal is cast around the end of the fuel pins held in the assembly. The fuel assembly is then lowered into the electrolyte liquid contained within an electrochemical cell with the cast metal block uppermost. During the operation of the electrochemical cell, the fuel pins dissolve into the electrolytic liquid and, as this happens, the fuel assembly may be further lowered into the cell until it is essentially all consumed.
The metal block may be formed from stainless steel which is the material used at that end of the fuel assembly having guide nozzles for regulating water flow through the assembly. The block of metal may typically extend for a length of about 10 cms.
By utilising a cast metal block at one end of the fuel assembly, electrical contact can then be simply made to the cast block itself which in turn connects to each fuel pin. In general the material used for casting must be electrically conducting. It should also, in the particular case of oxidised fuel assemblies, melt at a high enough temperature for the diffusion of oxygen from the oxide layer on the outside of the fuel pins into the molten melt to occur at a fast enough rate. However the melting temperature should not be so high that embrittlement or melting of the cladding itself takes place.
In a particular embodiment of the invention stainless steel is melted in a graphite crucible to achieve the appropriate balance of melting temperature and reducing conditions since some of the graphite dissolves in the stainless steel giving it a xe2x80x9creducingxe2x80x9d nature. In another embodiment, a higher carbon content steel is used, thereby removing the need for a graphite furnace.
The temperature to which the molten metal is raised is chosen to give good fusion bonding between the metal structure and the molten metal but with the avoidance of a temperature which is so high that embrittlement takes place or, in extreme cases, penetration of the metal structure by the molten metal. In the case where the molten metal is stainless steel which is melted in a graphite crucible and the metal structure is a fuel assembly including Zircaloy cladding, the temperature was between 1350xc2x0 and 1420xc2x0 C. preferably between 1375xc2x0 and 1395xc2x0 C. and most preferably about 1385xc2x0 C.
Preferably, the metallic structure is substantially fully immersed into the molten metal prior to solidification thereof. Preferably, the molten metal is then cooled at a rate of at least 50xc2x0 C. minxe2x88x921, more preferably at least 100xc2x0 C. minxe2x88x921 and most preferably about 200xc2x0 minxe2x88x921.